Lignin is the second most abundant plant-based biopolymer after cellulose. It is mostly found in plant cell walls and is an important structural component of plants due to its physical strength. Chemically, lignin is made of a random polymeric network composed of phenylpropane groups. Three monomeric units include coumaryl alcohol, coniferyl alcohol and sinapyl alcohol.
Those three monomeric units undergo a biosynthesis process to form a lignin's polymeric structure. The biosynthesis polymerization yields three types of segments within lignin: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S). Different plant sources yield different lignin structures because of the difference in concentrations of H, G, and S. For example, lignin from gymnosperms, a type of pine, is comprised largely of G-units with small amount of H-units. Unlike gymnosperms, angiosperm dicots, including many hardwoods, have a mixture of G-units and S-units, which reduce branching concentrations and can improve lignin's processing capability.
FIG. 1 shows the chemical structure of lignin. It has a complex three-dimensional structure with various types of functionalities and covalent links. Lignin's important C—O linkages are the β-O-4, α-O-4 and 4-O-5 linkages and its important C—C linkages are the β-5, 5-5, β-1 and β-β linkage. Common functional groups in lignin include methoxyl, phenolic hydroxyl, aliphatic hydroxyl and other carbonyl groups.
Smart polymers are useful polymeric materials because they may be self-healing, responsive to external stimuli, or exhibit shape-memory. Stimulus responsive polymers can change conformation upon receiving external stimuli, such as temperature, pH, light, electricity, magnetic field, and mechanical forces. Stimuli responsive polymers are used for a variety of applications including as sensors, drug delivery, tissue engineering, and reconstructive polymer structure.
Self-healing polymers have the ability to heal damage on a bulk structure either by an external stimulus or by spontaneous healing (autonomic healing). Most self-healing mechanisms are inspired from a healing property of natural organisms.
Shape memory polymers alter their dimensions in response to an applied stimulus. For example, the initial shape of a polymer can be temporarily changed with an applied stimulus, but the polymer can deform back to its original shape by applying the same initial stimulus. The shape memory effect can be repeated multiple times and the effect can be designed or predicted depending on the desired applications.
Although lignin is a highly stable material due to its densely packed aromatic groups and high molecular weight, lignin has been underutilized in modern materials. Lignin forms narrowly distributed nano-size beads in solution. From the lignin nanoparticles, hybridization of lignin with synthetic polymers has been shown to be an effective nanocomposite material. Also non-transition metal catalyzed chemical modifications of lignin are an inexpensive method of generating chemically convertible hydroxyl groups for polymeric modification on the surface of the lignin nanoparticles.
The main problems with making lignin-containing materials are: (1) the art's understanding of useful functional groups on lignin is not very advanced; (2) the technology useful for integrate lignin and petroleum-based polymers is not very advanced; (3) the synthesis techniques for well-defined polymers that can be integrated with lignin to make lignin-containing polymer products is not very advanced; (4) there has been little research devoted to combining lignin and petroleum-based polymers; and (5) lignin products are targeting narrowly to the cheap and low quality commodities market.